1. Field of the Invention
The present invention relates to an electro-photographic image forming apparatus such as a copier, a electrostatic printer, and a facsimile, and relates to a control method therefor.
2. Description of the Related Art
Generally, an image forming apparatus using an electro-photography process has a heating-type fixing device. This kind of image forming apparatus develops an electrostatic latent image formed on a photosensitive member by photoirradiation with developing material (toner) to form a toner image, transfers the toner image onto a recording medium such as a recording paper, and then, fixes the toner image thermally on a recording paper by the fixing device.
The fixing device is provided with a heating roller that is made by forming a resin coating on a surface of a cylindrical metal core, and a pressure roller having an elastic layer on a surface thereof that comes into contact with the heating roller while giving pressure thereto. A fixing process is performed under a condition where a surface temperature of the heating roller has reached to a predetermined fixing temperature. That is, a recording paper that bears a not-yet-fixed toner image is passed through a nip position between the heating roller and the pressure roller so that a toner-image side comes into contact with the heating roller. As a result, the toner image is fused and fixed to the recording paper by heat and pressure.
When the image forming apparatus starts after turning a power on, a predetermined electric power is applied to the heating roller to increase a temperature, while being monitored by a thermistor etc. so as to reach a fixing temperature of around 200° C. within a predetermined time. In order to compensate a quantity of heat taken away by the passing recording paper during an image formation of the image forming apparatus and to maintain the fixing temperature, an electric power corresponding to a difference between the temperature of the heating roller and a target temperature has to be applied. Since the temperature of the pressure roller that faces to the heating roller does not reach the fixing temperature immediately after starting the image formation, an electric power that is larger than the electric power corresponding to the quantity of heat taken away by the recording paper has to be applied. A required electric power is gradually reduced as the temperature in the fixing device including the above-mentioned pressure roller etc. increases to approach the fixing temperature due to continuation of a subsequent operation.
Thus, the fixing device needs a maximum fixing electric power when starting up after turning the power on and when starting the image formation immediately after that. In this case, since the image taming apparatus obtains the electric power from a general commercial power source via an electric power cable from a house wiring or an output port of the commercial power source (referred to as a plug socket, hereinafter) to the image forming apparatus, impedance of the electric power cable reduces a power source voltage at a power source input unit of the image forming apparatus correspondingly. When used in Japan, if a voltage falls 15% or more in general with respect to a nominal 100V input, the apparatus tends to generate malfunction etc. Therefore, Japanese laid-open patent publication (Kokai) No. 2004-226888 (JP2004-226888A) discloses a method to reduce an electric power consumption of an image forming apparatus by reducing a speed of an image formation process in proportion to a voltage drop. Moreover, Japanese laid-open patent publication (Kokai) No. 2007-102008 (JP2007-102008A) discloses a method to reduce a voltage drop by mainly controlling a fixing electric power within a range between upper and lower limits.
However, the voltage of the commercial power source fluctuates due to an external environment such as an area in which the image forming apparatus is installed and a house wiring. Moreover, the voltage of the commercial power source fluctuates due to an external factor such as an operating condition where a plug socket is shared with another device whose electric power consumption is large. That is, even when an electric power consumption of the image forming apparatus is very small, for example, when an electric power switch is OFF, the voltage of the commercial power source might have already dropped close to a lower limit voltage.
FIGS. 6A and 6B are graphs showing relations between the electric power consumption of the image forming apparatus and an input voltage of the commercial power source. FIG. 7 is a graph showing a relation among the electric power consumption of the image forming apparatus, the input voltage of the commercial power source, and a fixing unit temperature.
As shown in FIG. 6A, when the electric power consumption P of the image forming apparatus is a very small value less than a minimum electric power P2, the voltage drop amount of the input voltage Vac is very small with respect to a normal voltage V1(=100V). Then, if the input voltage Vac does not drop to a guaranteed operation voltage V2 (=85V) that can guarantee an operation, even when the electric power consumption P increases to a maximum electric power P1, the apparatus can operate normally.
However, as shown in FIG. 6B, if the input voltage Vac drops close to 85V due to the external factor even when the electric power consumption P is very small (601), there is a fear that the input voltage Vac becomes lower than the guaranteed operation voltage 85V as the electric power consumption P increases due to an application of the fixing electric power etc. afterward.
Accordingly, even if a user tries to operate the image forming apparatus from a minimum fixing electric power that can secure a fixing function under the above-mentioned condition like Japanese laid-open patent publication (Kokai) No. 2007-102008 (JP2007-102008A), the following problems arise. That is, since the electric power consumption of the image forming apparatus causes the voltage drop, there is a fear to cause an abnormal operation of the image forming apparatus or another device that shares the plug socket.
Even when the voltage drop is detected during the image formation, the voltage drop due to the external factor cannot be resolved by lowering the fixing electric power to the minimum fixing electric power. Therefore, when continuing a print operation, i.e., when continuing an application of the fixing electric power, there is a fear to cause the abnormal operation of the image forming apparatus or another device that shares the plug socket.
On the other hand, when a factor of the image forming apparatus causes the voltage drop that can be resolved by lowering the fixing electric power, the following matters can be shown. That is, when the electric power consumption P increases to the maximum electric power P1 as shown in a section 603 in FIG. 7, the input voltage drops to the guaranteed operation voltage V2 (=85V), which results in the abnormal operation. However, if the electric power consumption P is lowered as shown in sections 604 and 605, a margin of the input voltage Vac with respect to the guaranteed operation voltage V2 (=85V) increases. Therefore, if the electric power consumption P is controlled so as not to exceed a limit electric power P3 smaller than the maximum electric power P1 at the maximum for example, the operation can be continued.
Although a fixing unit temperature Tfix becomes lower than a target temperature (=T1) because of a reduction of the maximum electric power, it is unnecessary to lower a paper conveying speed when the fixing unit temperature Tfix is higher than a lower limit temperature T2 at which the fixing function can be ensured.
On the other hand, since the method to reduce the electric power consumption by lowering a print speed in proportion to the voltage drop as shown in Japanese laid-open patent publication (Kokai) No. 2004-226888 (JP2004-226888A) immediately reduces a number of sheets processed by the image formation, it brings a user a disadvantage.